The selection of materials used in bearings and other support members depends on an optimum matching of material properties with the requirements of low friction characteristics and high load-bearing capability.
Bearings are fabricated in a variety of ways, including casting, machining and powder metallurgy methods. Most bearings useful in heavy machinery are used in conjunction with a lubricant or general oil to reduce friction. These oil film-bearing materials include lead-tin alloys ("babitt metals"), copper-lead alloys, bronzes, aluminum, cast-iron, brass and steel, as well as commercial resins.
Another popular type of bearing material is boundary-lubricating (or "self-lubricating") bearing material. This type of material is useful under dry or sparsely lubricated conditions. Some of the more commonly used materials are oil-impregnated porous metals, wood, rubber and graphite by itself or mixed with resins, copper, babbit, silver or oil. These materials permit design simplification, low maintenance and freedom from oil contamination. They perform best at low speeds and intermittant operations.
Bearings of graphite, in particular, may be used with low viscosity liquids and in dry operations. Graphite's usefulness in dry operations stems from the ability of the graphite planes to slip past each other, which is due to the presence of water vapor or oxygen in graphite. Therefore, if conditions are such that the water or oxygen is driven off (i.e. elevated temperatures, high altitudes), the antifriction properties of the graphite bearings are impaired.
Bearings have been prepared from compression molded graphite-containing compacts filled with resins, copper, oil, etc., see Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Ed., Vol. 3, pg. 683 (1978). These graphite bearings have found applications, for example, in machinery such as food or drug processors where contamination by oil and grease must be avoided and in chemical pumps which operate on low-viscosity, non-lubricating fluids. While graphite is a satisfactory, dry lubricant at room temperature in an ordinary atmosphere, at higher temperatures or altitudes, the water vapor leaves the system and the anti-friction properties are impaired. In addition, graphite compositions are characterized by brittleness and a low coefficient of expansion, which impart a tendency to crack when such compositions are struck on an edge or subject to high thermal, tensile, or bending stresses.
Thus, it has been necessary to confine application of bearings and other support members containing graphite within a given range of temperature and stress. It has also been required, in the fabrication of graphite bearings, to avoid sharp corners, thin sections and the like, and to relieve edges with a chamfer. It has further been the practice to surround graphite bearings with a steel sleeve to provide mechanical support and minimize changes in shaft clearance with temperature variations.
Therefore, a bearing having the low-friction properties of graphite but with improved strength and temperature resistance is desirable. In addition, the need often arises for a member that is not only self-lubricating but also electricly conductive. Examples of such members are small to medium machine gears and bearings used in printers, cash registers and the like.
Accordingly, it is desirable to develop a bearing or other members which has low-friction characteristics, which may conduct electricity as well, which is easy and economical to manufacture, and which is sufficiently strong to reduce the need for metal reinforcing.
It is therefore an object of this invention to provide an improved bearing or other support member made from graphite.
It is another object of this invention to provide a support member containing graphite which is stable in air and over a wide range of temperatures.
It is another object of this invention to provide a support member containing graphite which has superior low-friction characteristics.
It is a further object of this invention to achieve superior load-bearing capability due to higher strength and greater edge-holding properties.
It is another object of this invention to provide a contact that exhibits low friction properties and conducts electricity simultaneously.
It is still another object of this invention to provide a more efficient method of manufacture of a graphite containing support member which requires simplified dies for pressing.
Still another object of this invention is to provide a graphite support member requiring reduced amounts of metal reinforcement.